Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to electronics and, more particularly, to the packaging and fabrication of electronic devices.
2. Description of the Background
DC-to-DC power converters are widely utilized in power supplies to convert an input DC voltage into a specified output DC voltage. FIG. 1 is a schematic diagram of one type of DC/DC forward converter 10. The converter 10 includes a DC voltage source 12, a primary switch 14, a transformer 16, and an output circuit 18. The transformer 16 includes a primary winding 20, a secondary winding 22, and a magnetic core 23. The primary switch 14 is coupled to the primary winding 20 of the transformer 16 such that when the primary switch 14 is closed, the DC voltage from the DC voltage source 12 is coupled across the primary winding 20. The converter 10 is referred to as a forward converter because power flow is gated by the primary switch 14 and energy is transferred forward, i.e., from the primary winding 20 of the transformer 16 to the secondary winding 22, during the conductive or ON period of the primary switch 14.
The secondary winding 22 of the transformer 16 is coupled to the output circuit 18 of the converter 10. The output circuit 18 includes a first rectifier 24, a second rectifier 26, an inductor 28, and a capacitor 30. When the primary switch 14 is closed (i.e., when there is a positive voltage across the primary winding 22), current induced on the secondary winding 22 by the primary winding 20 flows through the first rectifier 24. The current flows through the load 32 connected to the output terminals 34 of the converter 10 and back to the secondary winding 22. Conversely, during non-conductive or OFF time period of the primary switch 14 (i.e., when the primary switch 14 is open), the load current (IL) flows through the second rectifier 26, and the current in the secondary winding 22 of the transformer 16 is essentially zero.
The inductor 28 and the capacitor 30 of the output circuit 18 form a filter to provide averaging and smoothing of the output voltage (i.e., the voltage across the output terminals 34) during the ON and OFF periods of the primary switch 14. The inductor 28 and capacitor 30 maintain the load current IL substantially constant during these time periods.
In the forward DC/DC converter industry, to ensure interoperability with power supplies from other manufacturers, converters are often fabricated according to a standard size, referred to as a brick, which is 4.6 inchesxc3x972.4 inchesxc3x970.5 inches. Moreover, as the computer and communications industry continue their migration towards lower voltage logic, the DC/DC converter market is also driving towards higher power densities at lower costs. Conventionally, forward converters are fabricated by mounting individually packaged components, such as the transformer 16 and the rectifiers 24, 26 to a thermally and electrically conductive substrate. The packaging, in conjunction with the lead terminals for the components, adds considerably to the respective sizes of the components, and thereby limits how densely the individual components of the converter may be situated. For example, for the forward converter 10 shown in FIG. 1, the transformer 16 and the rectifiers 24, 26 each comprise individually packaged components, and the distance between each component, and hence the density of the converter 10, is limited by the packaging and lead terminals of the respective components.
Furthermore, the efficiency of the converter 10 is adversely affected by stray inductance introduced into the converter 10. Inductance in the converter 10 may be caused by any loop of current, and is proportional to the area of the loop. One solution to minimize the amount of stray inductance in the converter 10 is to physically locate the components of the converter 10 that form the loop close together, subject to other circuit constraints.
For example, as discussed hereinbefore, the load current IL of the converter 10 illustrated in FIG. 1 flows alternately through the first and second rectifiers 24, 26. During the changeover periods, the current decreases in one rectifier 24, 26 and increases in the other. The rate of change of the current in the rectifiers 24, 26 is limited by the stray inductance in the current path through the rectifiers 24, 26 and by the leakage inductance of the secondary winding 22 of the transformer 16. At the common cathode connection of the rectifiers 24, 26, and in the inductor 28, the current is essentially constant. In the path through the rectifiers 24, 26 where current changes rapidly, however, the stray inductance adversely affects the performance of the converter 10 and stores energy that may have to be dissipated in snubbers (not shown), thereby reducing overall efficiency. It is thus desirable to limit the inductance in this path of rapidly changing current. However, as discussed hereinbefore, the density of the components of the converter 10 is limited by the fact that the components are individually packaged and leaded.
Accordingly, there exists a need for a high-density converter circuit which minimizes the inductive loop between components to reduce the introduction of stray inductance into the converter.
The present invention is directed to an electrical device. According to one embodiment, the electrical device includes a first substrate, a second substrate facing the first substrate, an unpackaged semiconductor die electrically and thermally connected to the first substrate, a first lead electrically connected between the unpackaged semiconductor die and the second substrate, and a second lead electrically connected between the first and second substrates. The first substrate may be electrically conductive, such as an insulated metal substrate. The second substrate may be a circuit board. The device may include a first conductive tab electrically and thermally connected between the unpackaged semiconductor die and the first substrate. The device may also include a second conductive tab electrically and thermally connected to the unpackaged semiconductor die and electrically connected to the first lead. According to one embodiment of the present invention, the unpackaged semiconductor die includes an unpackaged silicon die, and the first and second conductive tabs include molybdenum. In addition, the first lead may include a strain relief portion.
According to another embodiment, the present invention is directed to a converter including a first substrate, a second substrate facing the first substrate, an unpackaged rectifier electrically and thermally connected to the first substrate, a first lead electrically connected between the rectifier and the second substrate, a second lead electrically connected between the first and second substrates, and a transformer electrically connected to the second substrate. According to one embodiment, the rectifier is mounted to the first substrate in a position vertically above the position of the transformer on the substrate.
The benefits of the present invention may be utilized to realize converter circuits having a reduced inductance path between components, such as the output rectifiers and the transformer, thereby minimizing the amount of stray inductance introduced into the converter in comparison with conventional converter circuits including individually packaged components. In addition, by removing the packaging associated with the rectifiers of a conventional forward DC/DC converter, a converter utilizing the benefits of the present invention results in approximately twice the die area for the output rectifiers for the same footprint area of the substrate, therefore reducing costs and improving power conversion density. These and other benefits of the present invention will be apparent from the detailed description of the invention hereinbelow.